In many industries, the blending of particulate material, for example, powders is often critical to the performance or desired characteristics of the resulting product, for example, the blending of powders to make concrete, the blending of pharmaceuticals, the blending of food ingredients, or the blending of ceramics, among other products. However, the blending equipment typically used to blend these and other particulate materials typically have the disadvantages of producing lack of uniform particle distribution in the product, sensitivity to particle size, and the excess generation and loss of fine particles (or “fines”) in the product. Aspects of the present invention employ pressurized fluid flows through conduits, for example, “draft tubes,” to mix, treat, coat, and otherwise handle particulate materials.
One prior art type of particulate mixer is known as a “pneumatic blender.” Pneumatic blenders are similar to fluidized bed mixers in that they use air or a gas to agitate the granular or particulate material to produce a particle mixture. Pneumatic mixers are effective for blending products that do not require uniform particle distribution. For example, pneumatic blenders are typically not used for mixing pharmaceuticals that typically require a somewhat uniform particle distribution in the resulting product. Pneumatic blenders are effective for blending component particles that are similar in size, density, and shape. Pneumatic blenders are, however, highly scalable and can be used to blend particles of vastly differing size and shape. One disadvantage of pneumatic blenders is that they typically carry off the finer particles from the mixture and, as such, need some form of filtration device, or “bag house,” downstream of the blender to prevent air pollution or loss of product.
Another prior art mixing device is a “convection-type blender.” Convection blenders include ribbon, plow paddle, and conical orbiting screw mixers, among others. A convention paddle blender is similar to the type of mixer used for blending ingredients for a cake. Typically, paddle mixers produce high shear in the particulate or powder during blending. One disadvantage of the paddle blender is that areas of stagnation can occur where the material fails to mix properly or remains unmixed, for example, in regions near the wall of the container in which the particles are mixed. Other disadvantages of convection mixers include the poor blending of mixtures where at least one component is very dilute. Also, convection blenders are recognized in the art as poor blenders of powders that are very dense or very abrasive. Convection blenders are also typically difficult to clean, are difficult to scale up due to their power requirements, provide inconsistent product, and can be characterized by excessive wear due to abrasion.
Another prior art particulate blending device is the “diffusion-type blender.” Diffusion blenders operate by allowing the particles to be blended to move with respect to each other by moving the actual containment vessel itself. Diffusion blenders are often called “tumble blenders” because they resemble a container that is tumbled in some fashion. Diffusion blenders can accommodate particles that are vastly different in size, density, and total concentration. These systems are easily scaled to huge sizes and can be customized to accommodate different types of materials.
Aspects of the present invention overcome many of the disadvantages of the prior art blending, mixing, and treating devices while providing improved blending, mixing, and treatment of particulate material
One industry that can benefit from aspects of the present invention is the mortar or cement industry. The need to produce roads and bridges with concrete structures and surfaces that are stronger, more durable, and less costly to maintain is imperative. To improve the performance of concretes in these structures, recent compositions have included fly ash and condensed silica fume. These materials increase the strength of concrete, reduce its permeability, and have the potential to decrease cracking through improvements in the paste aggregate bond.
However, most of these fine particles, particularly the silica fume, exist in the form of fine spheres linked together into clusters, rather than as isolated spheres [St. John, et al. (1995)]. The performance gains from using materials like silica fume are primarily related to the chemical reaction between calcium hydroxide and the fine material, and secondarily due to the improved particle packing density resulting from the uniform incorporation of finer and finer particles into the mix [Lange, et al. (1997) and Chengzhi, et al. (1996)]. Diamond, et al. (2004) point out that most silica fume used in concrete is in the dry, densified form and consists of agglomerates of sizes between 10 μm and several millimeters. Lagerblad, et al. (1995) have reported that granulated condensed silica fume is not easily dispersed. In conventionally mixed concrete, the breakdown of densified silica fume agglomerates is incomplete and a portion of the agglomerates remains at least partly intact. Undispersed agglomerates in mortars and concretes result in poor performance gains due to the inability of the finest size fraction of the particles to effectively enter the interfacial transition zone.
Dispersing fine particles in cement is normally achieved in the liquid phase using surfactants known as superplasticizers [Hooton, et al. (1998)]. These admixtures have long been used to help disperse the cementitious powder but the dispersive action occurs only after water is added and the ‘polymerization’ (hydration and micro-crystalline interlocking) reactions begin [Anderson, et al. (1988 and Ferraris, et al. (1992)]. Scrivener (1989) reported that despite the use of superplasticizer, some clumps of silica fume are still present and so the material is not used as efficiently as it could be.
Another approach to providing better dispersion of the agglomerates is to take advantage of the recent advances in dry-phase processing techniques [Iwasaki, et al. (2001)]. These techniques provide the means to transform the mechanical properties of the cement by dispersing the powder uniformly in very small clumps prior to hydration. The addition of fine particles in coarser ones improves the fluidization characteristics of the coarser material [Haberko (1979)] by dispersing fines into the voids between the larger particles and reducing the channeling and bubbling of the fluidizing gas [Matsumoto, et al. (1986)]. The mixture prevents a cohesive powder such as cement from behaving as a ‘weak’ solid, held together by chemical and electrostatic forces. Without the addition of the large particles, the powder would crack causing channeling of the gas to take place, rather than aeration and mixing of the particles [Kendall, et al. (2001)].
The inventors surmise that a dry mechanical dispersion of powders should lead to a more uniform mixture with smaller clumps of material and would serve as a precursor to chemical dispersants, such as superplasticizers, allowing the dispersants to work more effectively, since the diffusion length required to get to the center of a particle clump will be reduced. Unfortunately, conventional concrete or mortar mixing equipment cannot provide the intensity of agitation necessary to effectively mix and disperse the finest particles [Ferraris, et al. (2001)]. Thus, obtaining a uniform mixture of these components is generally difficult, inhibiting performance gains and increasing the cost of the materials. Further, the inventors surmise that the dry premixing process, if executed correctly, should be able to produce mortars with properties comparable to the best, high-shear rotary mixers, but at much higher throughputs than are possible with rotary mixers alone.
The “draft tube spout fluid bed” (DTSFB) mixer is also known as an effective mixing device. Littman (1996) summarized the state of development of the DTSFB mixer. U.S. Pat. Nos. 5,248,222 and 5,254,168, both of Littman (one of the co-inventors of the present invention), et al. (the disclosures of which are included by reference herein) disclose advancements in the particulate mixing art that can be achieved with the DTSFB mixer.
Plawsky, et al. (2003) reported that the dry premixing of sand and cement using a first-generation, DTSFB mixer was more effective as the cement content was reduced and that it might be possible to produce commercially acceptable mortar with lower cement content. However, a considerable amount of cement fines passed through the cyclone separator of Plawsky, et al. (2003) and ended up, unincorporated, in a bag house filter unit. Due to this loss, the early strength gain of the initial mixtures was slower than the control samples even though the long-term strengths of the dry, premixed and control samples were comparable. The inventors now surmise that the loss of fine particles may significantly affect mortar performance particularly when ultrafine particles, such as fly ash and silica fume powders, are added to the mixture. In an attempt to avoid the disadvantages of this and other prior art, for example, to insure more complete incorporation of all materials, the inventors designed, tested, and developed the present invention in its many aspects.
Aspects of the present invention overcome the above disadvantages and other disadvantages of prior art particulate material blending devices.